This invention relates to the manufacture of wire-scribed interconnection circuit boards. More particularly, this invention relates to the use of laser energy for bonding and stripping conductors during the scribing process.
U.S. Pat. No. 3,674,914 to Burr, No. 3,674,602 to Keogh and Canadian Pat. No. 1,102,924 granted June 9, 1981 describe a process and apparatus for scribing conductors onto an insulating substrate which has been precoated with an adhesive layer by continuously feeding a fine insulated wire onto the surface of the substrate, affixing the conductors to the substrate and cutting the conductors at predetermined points. The process described in these patents is capable of producing a conductor image of a predetermined interconnect pattern.
The conductor pattern is formed by tacking the insulated wire to the adhesive layer by means of a scribing element which locally heats the adhesive layer. The adhesive melts, is formed to at least partially surround the conductor and then, upon cooling, creates a bond between the conductor and the substrate. The adhesive material thus captures the conductor and forms a combined mechanical and chemical bond.
European Patent Application No. 110,820 published July 25, 1984 describes an apparatus for scribing wire into an activated layer of photocurable adhesive. The adhesive is activated using ultrasonic energy and subsequently cured by means of optical energy which may be laser energy.
Japanese Laid-Open Application No. 57-136,391, published Aug. 23, 1982, describes a process wherein wires are scribed in a predetermined pattern onto an adhesive-coated surface of a substrate. Laser energy in combination with ultrasonic energy is used to soften the adhesive and press the wire into the adhesive as the wire is being scribed.
There are, however, many problems and obstacles faced in using a laser beam as an energy source to activate an adhesive used in the scribing process, whether the adhesive is located on the surface to be scribed or on the conductor. Directing laser energy to the adhesive as a continuous beam achieves a consistent level of activation along the predetermined conductor pattern only if the laser beam is moved along the adhesive at a constant velocity or modulated according to complex control variables. The energy received by a particular section of the adhesive increases as the duration of its exposure to the energy increases. The duration of exposure is inversely proportional to the velocity at which the laser beam moves along the predetermined pattern on the adhesive. Thus, if the speed at which a conductor is scribed changes (also referred to as scribing speed), the exposure time of the adhesive in different area also varies. During the scribing process, the scribing element can change the scribing speed under at least three circumstances: when the scribing element accelerates from a stop to a predetermined running speed; when an inflection or turn is made (requiring the scribing element to decelerate before making a turn, to stop in order to make the inflection, and to accelerate to the running speed after making the turn); and when the scribing element decelerates to a stop at the end of a conductor run. As a result, when the speed of the scribing element is lower than the running speed, the adhesive may receive excess energy causing the adhesive or surrounding materials to be scorched or damaged. If the level of laser energy is lowered to compensate for the excess energy at lower speeds, the amount of energy received by other sections of adhesive at running speed may be insufficient to activate the adhesive.
It is, therefore, an object of this invention to provide method and apparatus for scribing conductors in a predetermined pattern onto a substrate using a precisely-controlled laser beam or beams to activate the adhesive used to bond the conductors to the substrate.
It is a further object of this invention to provide a method of constructing a wire-scribed circuit board wherein the the bonding adhesive activation is controlled in a simple manner.
It is yet a further object to provide method and apparatus for effective adhesive bonding and wire stripping of scribed conductors using one or more laser beams.